1. Field of the Invention
The present invention relates to a fuel cap having a pressure regulating valve to regulate the inner pressure of a fuel tank, and more specifically to a mechanism of improving the flow rate characteristics of the pressure regulating valve.
2. Description of the Related Art
A known fuel cap has a cap main body screwed to an inlet of a fuel tank, and a pressure regulating valve received in to the cap main body to regulate the inner pressure of the fuel tank (see, for example, JP-A-10-278958 and 2002-347454). The pressure regulating valve includes a positive pressure valve and a negative pressure valve. The positive pressure valve opens when the inner pressure of the fuel tank rises over a preset positive pressure level. The negative pressure valve opens when the inner pressure of the fuel tank lowers below a preset negative pressure level. The pressure regulating valve accordingly regulates the inner pressure of the fuel tank in a preset pressure range.
The increased negative pressure has a high potential for damage of the fuel tank. The negative pressure valve is thus required to quickly open under the negative pressure condition and make a high flow rate of the outside air for cancellation of the negative pressure condition in the fuel tank.